I don't understand this but put down my best attempt at understanding why down below. After it I've included what chatgpt said about the matter.

OK I understand why angular momentum is significant for turning galaxies into discs It's because you use the demo in physics class of a guy holding out hammers on her rotating chair now focus on how much heavy mass is outside extended and then think or focus primarily on how much energy it takes to get that mass moving in a rotation and everything is constant here but you had to put a lot of energy into spinning the mass and now you are when he retracts his arms it focuses the energy over a smaller amount of space

The reason why I was confused

Angular momentum describes how things speed up with a constant amount of energy and mass while rotating when you decrease the amount of space these things have to operate over... you are focusing all of these things over a smaller amount of space.

I think that the Galaxy while it's spinning does not have angular momentum contribute directly too flattening the shape. I think that it's more about disallowing it from returning to a less flattened shape. Which is indirectly contributing it to become a flattened shape.

Because as it was stated before as a factor contributing to the flattening of the Galaxy I had to try to imagine pushing the astral bodies unto one plane via angular momentum. This was harder to do because in space you don't have to compete with atmosphere which is how angular momentum made sense to me at the time. Which is a little bit of Tangent to my explanation here. But maybe not really. I guess what I am imagining right now is that it's harder for a faster object rotating to return to a less flattened shape, more slowly moving, than how it is at that faster velocity

Me: why does conservation of angular momentum matter in space to flatten the shape of galaxies

Chat gpt:

Conservation of angular momentum is a crucial factor in shaping the structure of galaxies, particularly in the context of the formation and evolution of disk-like structures such as spiral galaxies. This principle plays a significant role in flattening the shape of galaxies and determining their overall rotational properties. Here's how conservation of angular momentum applies to galaxy formation:

1. **Initial Rotation:** In the early universe, regions of space that eventually give rise to galaxies may have tiny initial rotations or angular momenta. As matter within these regions collapses under the influence of gravity, this initial angular momentum becomes more pronounced.

2. **Collapse and Conservation:** As gas and dust particles within the collapsing region come closer together, their total angular momentum remains conserved. This means that if the initial region had even a slight amount of rotation, this rotation will become more concentrated as the material collapses. As a result, the collapsing gas and dust tend to form a spinning, flattened disk-like structure.

3. **Formation of a Disk:** The conservation of angular momentum leads to the formation of a rotating disk. Particles that have more angular momentum end up in orbits that lie closer to the equatorial plane of the disk. This process is similar to how a spinning ice skater speeds up when they pull in their arms, conserving their angular momentum.

4. **Star Formation:** Within the disk, gas and dust particles continue to coalesce and eventually form stars. The rotation of the disk is inherited by the stars, causing them to follow similar orbits within the plane of the galaxy.

5. **Stable Galactic Structure:** The flattened, disk-like structure that arises from the conservation of angular momentum helps to stabilize the galaxy over time. The gravitational forces between stars and other matter in the disk act as centripetal forces, counteracting the tendency for the material to collapse directly toward the galactic center.

6. **Spiral Arms and Evolution:** The combination of rotation and gravitational interactions can lead to the formation of spiral arms in some galaxies. These arms are regions of enhanced star formation and are often a result of the complex interplay between gravitational forces and density waves within the rotating disk.

In summary, the conservation of angular momentum is a fundamental principle of physics that influences the shape and structure of galaxies. It leads to the formation of flattened, rotating disk-like structures and contributes to the overall stability and dynamics of galaxies over their lifetimes.
  • $\begingroup$ You should avoid AI-generated answers for physics-related questions. Not only are they useless, but they aren't welcome here. Either just ask your question, or show actual research. $\endgroup$
    – Miyase
    Commented Aug 7, 2023 at 7:20
  • $\begingroup$ I concur with contributor Miyase. My position specifically: AI-generated material can possibly be of some use in the course of personally exploring a subject, but it should not in any form be submitted here. $\endgroup$
    – Cleonis
    Commented Aug 7, 2023 at 9:44

2 Answers 2


Disk galaxies are that way because most star formation occured in gas that had flattened to a disk first.

Such galaxies could have started with a much more spherical gas distribution which then flattens to a disk. An orbiting blob of gas can lose kinetic energy by interacting with other gas clouds - generating heat and then radiation. The radiation carries away kinetic energy but not much angular momentum - hence the angular momentum of the gas is approximately conserved. As a result, the ratio of angular momentum to kinetic energy increases and the shape which maximises the ratio of angular momentum to kinetic energy (in some appropriately dimensionless units) is a rotating disk.

Note that spherical distributions of "dissipationless" objects (e.g., point-like stars) do not flatten to the same extent. That is why the first generation of stars in our Galaxy, born from the initially spherical gas cloud, are still distributed in a roughly spherical halo.


You ask:

How does the conservation of angular momentum contribute to the flattening of a galaxies shape

An early model:

The distribution of matter in the early universe was in clumps that consisted mostly of dark matter. These clumps interacted gravitationally, putting tidal torques on each other that acted to give them some angular momentum. As the baryonic matter cooled, it dissipated some energy and contracted toward the center. With angular momentum conserved, the matter near the center speeds up its rotation. Then, like a spinning ball of pizza dough, the matter forms into a tight disk

Italics mine.

Once the clump of matter acquires an angular momentum, conservation of angular momentum imposes the disk shape.

  • $\begingroup$ when you say angular momentum I just think of how this is given a value that stays constant for the entire system. I still can't imagine how that manifests as specifically a disc shape. I can imagine a spherical cloud around the entity which is intuitive $\endgroup$ Commented Aug 11, 2023 at 17:31
  • $\begingroup$ I repeat the quote above "With angular momentum conserved, the matter near the center speeds up its rotation. Then, like a spinning ball of pizza dough, the matter forms into a tight disk" $\endgroup$
    – anna v
    Commented Aug 11, 2023 at 18:56

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